Attn Twin Reverb experts - questions about a 68 (pics)

[slack]

So I lugged home two Twin Reverbs this week. A 73 master volume and this 68 (warning, big pics).

I know it's not the AB763 circuit, but it is a non-master volume.

First, if any of you experts can make any comments about the circuit or components based on those pics I would certainly appreciate it. It doesn't appear molested to me, but I'm not an expert on the little mods, like Blackfacing, that many of you are.

Secondly, it was loaded with those extremely heavy Electro-Voice EVM 12L speakers, and the weight had broken one of the baffle mounts off the inside of the cab (pictures in link). Those things were just held on by two big staples. What would be a proper repair? Drill it for two screws?

I'll probably start another thread about what speakers I might put in this, but if you have an opinion, would Eminence Red Coat Red Fangs be alright wattage-wise? I happen to have two in a Bassman cab (and two lesser wattage Webers to go in that cab). I think this Twin is 100W. Those speakers are rated 30W rms/60W peak. I'm not rocking any stadiums, and in fact main use would probably be Rhodes piano, then Tele and lap.

Appreciate input and advice. Thanks.

[El Capitan]

I love those speakers! EVM 12L's.

I would drill the baffle mount and install wood screws, to remount it. Of course, measure and don't get too long of a screw, and I figured you knew that.

El

[Dacious]

That certainly appears very original: those bronze- and white-coloured Mallorys are all showing signs of leaking. One of the big bastards under the filter cap cover is cancerous, too. The others won't be far off.

That 6-diode full wave rectifier scheme I've seen on a mid-70's MV Vibroverb. They are original RCA diodes - will likely crap themselves if the filter caps get changed.

That is the mystical combined cathode/fixed bias model, with the big M-F 10 watt sandbath resistors tied to ground. Lasted less than a year.

I have not heard one - I had read it suggested that sonically the biasing scheme shown was not an enhancement on BF fixed biasing and many got converted to the newer S/F standard under warranty or before sale.

I would use a strong wood glue with screws to reattach, unless you are keen on 100% originality. Good news is it appears that is a pine cabinet.

I wouldn't run 60 watts of speaker in an amp designed for 100 watts output personally. It might be OK - but even at '6' on the dial you are pumping much more bass amplitude than they are designed for.

Celestion G12/75s don't sound bad in big SF amps, I've found. YMMV. Don't let anyone talk you out of the Sylvannias unless they test bad or are microphonic.

The EVs are worth a packet secondhand.

Just MO, but if the '73 is in good shape and plays well, it might be a better bet......

[slack]

I don't know a thing about those speakers, other than they weigh about 25 lbs each and I had to take them out just to carry the thing up some stairs by myself. Seriously.

What do you make of that date code? 69, 75, 79, or 85? Hmm.

[Dacious]

Those cast EVs (based on Altecs, I believe) didn't arrive until the seventies. I had a pair - I know what you mean. Serious sound guys still seek them out. Even blown, they will sell for the price of a new one minus the recone job.

I would say it's 45th week/1979 but I can't be sure.

[slack]

Originally Posted by Dacious

I wouldn't run 60 watts of speaker in an amp designed for 100 watts output personally. It might be OK - but even at '6' on the dial you are pumping much more bass amplitude than they are designed for.

I've got questions about some of your other comments, but regarding the speaker wattage: The amp outputs 100W, but that is divided among the two speakers, whether parallel or series. So the two Red Fangs should handle 120W peak and should be seeing no more than 50W each in that amp.

[slack]

Originally Posted by Dacious

That is the mystical combined cathode/fixed bias model, with the big M-F 10 watt sandbath resistors tied to ground. Lasted less than a year.

Hmm, Greg Gagliano, who wrote the many pages of info at Ampwares, says on one of those pages: "There is also an unconfirmed report of an early '68 Twin Reverb with the cathode bias circuit. If you see one of these cathode biased amps, please let me know!"

Not sure if this is one, but I don't think it's an early 68 Twin.

[Dacious]

The amp's output is 100w RMS, and granted, that's optimistic on a good day. Peaking, and depending on how you measure, that could reach 200 watts or more instant-instant. A normal speaker that is rated 50w RS and 100w peak will take that in its' stride.

If you compare the size of the spiders and voicecoil assemblies on the EVs with the Eminences you'll see what I mean.

It certainly wouldn't hurt to try them. If they show any signs of distress like flapping the cones or farting on low notes at moderate volumes I would change them out straight away. Keep an eye on bass settings - that's what'll hurt them.

FWIW, I've seen conflicting info on power of these. I read in some tech info that late BF and early SF were 85w only, but people on this board have sworn blue blind that is wrong and all were 100 watters.

[Dacious]

Hmm, Greg Gagliano, who wrote the many pages of info at Ampwares, says on one of those pages: "There is also an unconfirmed report of an early '68 Twin Reverb with the cathode bias circuit. If you see one of these cathode biased amps, please let me know!"

Not sure if this is one, but I don't think it's an early 68 Twin.

No doubt Mark Norwine or Tim Schwartz or Tremo will chime in sooner or later - I'd defer to them. I can't see what else you'd tie to ground through a 7w (sorry, not 10w), 150ohm resistor than pin 8, which is the cathode on a 6L6.

It doesn't appear on any schematic on the FFAG.

[slack]

With all the talk about non-master volume Twins, I'm surprised the 73 may be more desirable. It has the push-pull MV. I'd also think that no matter what this is, it could be easily modded to be good.

Btw, I have long had about as pristine a later 135W Twin as could possibly exist. But, one or both of these is gonna replace it.

[Tremo]

Yep, that's definitely the dreaded 68 circuit. Looks unmodded, but I do see a couple signal caps have been replaced. It definitely could use a cap job. I would also convert it to AB763.

Re the speakers, yeah those EVs weigh a ton, but they're excellent speakers. A pair of Webers with 60 oz magnets will be nearly as heavy.

[z8894]

It's the AC568 circuit. The one I'm working on had some of the bad stuff already cut out. I've removed the balanced bias. It even had the baffle mounts broken just like in your picture. On this one, not only were the mounts broken off but the screw holes in the baffle were all reemed out. I used MinWax heavy duty epoxy wood filler to repair the baffle and drilled new screw holes. I used wood glue and finish nails to reattach the mounts. It worked fine.
It had a raspy noise at low volume but rebuilding the phase inverter section cleared that up. This one has some Jensen Vibranto speakers in it, not original. I recapped it, replaced the screen and grid resistors, replaced a couple scratchy pots and put a new set of JJs in it. Sounds great now.

[Stan Martin]

get a cap job and have the right filter cap resistors in there. Also lose the caps on the power tubes.

As far as speakers go. I would use a minimum rating of 50 watts. The Celestion G12-75 is a good suggestion.
A pair of Alltones sound great or I think the Eminence equivalent is a Legend 125(?). The Eminence GB12 is a great sounding Twin speaker. Sort of a British meets American speaker vibe. Kenny Vaughan use to swear by the GB 12's.

I would buy a new baffle from Rodgers amps down in Florida. Although some people will say the baffle has no affect on your tone, it does. A/B an MDF baffle to an aged birch baffle. MDF is a tone suck.

[Tim Swartz]

A few weeks back there was a thread on fixed bias which led to a discussion on what it is and how it differs from cathode bias. These amps are both, I coined the name "Mixed bias" to define this scheme. You can simply lose the 150K cathode resistors and those caps on the side of the chassis and ground each cathode.... But, are there sonic advantages to this arrangement? I really don't see how it could be all that detrimental to good tone... In fact, could it be the best of both worlds? Cathode bias but with ability to fine tune current draw? Could "mixed bias" be the next buzzword used to sell boutique amps to a market that seems to live for buzzwords?

[slack]

If I'm properly interpreting the comments I've received here and elsewhere, this particular circuit can very easily be converted to the AB763. And that nothing has been buggered up and previous maintenence work looks good.

Any recommendations in in the LA area? I'd consider shipping as well, but just the chassis. In fact, I'd even prefer to just take the chassis to a shop if the tech is in SoCal if that's acceptable.

[Dacious]

I'd say that amp is almost 'no maintentance' - which is good, because it hasn't been jiggered with.

I made the comment about the '73 MV, because I've heard several early MV ones, and they sound just great even through the stock blue label speakers (so there's room to move) - plus although the '68 is not really 'better' it is more collectible - especially in this condition, due to the 'drip-guard'. Sometimes when you own something like that it feels awkward to make any changes.

The MV likely has a birchply cab. Just leave the boost unpulled - some people obsess about removing the dual pot which can also be done. I have also heard of people doing strange things like making it a pull-NFB adjuster.

If you like the '68 by all means get it - it's rarity factor is a plus (I'd hit it), as Tim says just because folklore says something is crap, don't mean it is. It may well break up earlier than other twins which could only be a good thing.

[slack]

Originally Posted by Dacious

If you like the '68 by all means get it - it's rarity factor is a plus (I'd hit it), as Tim says just because folklore says something is crap, don't mean it is. It may well break up earlier than other twins which could only be a good thing.

Dacious, I own both of these. The first thing I said was that I had just lugged both home. Got a good deal.

Was just taking some surface rust off the 68s kick-back legs. They say Patent Pending on the bottom, kinda cool.

I'm likely gonna part with my 77 135W Twin, and retain one or both of these as my monster amp (I haven't even pulled the chassis on the 73 yet to inspect). I intend to keep one attached to a Rhodes in the house.

[Tremo]

Originally Posted by slack

If I'm properly interpreting the comments I've received here and elsewhere, this particular circuit can very easily be converted to the AB763. And that nothing has been buggered up and previous maintenence work looks good.

Any recommendations in in the LA area? I'd consider shipping as well, but just the chassis. In fact, I'd even prefer to just take the chassis to a shop if the tech is in SoCal if that's acceptable.

Yes, very easy to convert to AB763. Yes, no buggering or tossing visible. Previous repairs look good.

Tech in LaLa land? Hummm..... I'm not real familiar with the SoCal scene, sorry. Doesn't Doug Rocketfartz still do repair work? Just don't let him mod the thing into a high gain buzz box. If you were in NorCal I'd say send it to me.

[Dacious]

Ah, sorry Slack. I thought you were tossing up between them.

I have to admit one of my 'should have bought its' is a stock MV 100 watter now owned by a friend of a friend. It had stock tubes and speakers, and totally original and good condition tolex, no scars. It sounded sweet.

The guy owned a music shop and didn't really want to sell so he overpriced it. This persons' wife was looking for the ideal 50th present, and he had 'always wanted a Twin' so she sprung the dollars.

[David Barnett]

Originally Posted by Tremo

Yep, that's definitely the dreaded 68 circuit. Looks unmodded, but I do see a couple signal caps have been replaced. It definitely could use a cap job. I would also convert it to AB763.

Is there enough B- voltage to pull down the grids for a conversion to AB763 full fixed-bias operation?

[Tremo]

Originally Posted by David Barnett

Is there enough B- voltage to pull down the grids for a conversion to AB763 full fixed-bias operation?

Yes.

[mark norwine]

Just when I thought I'd seen it all....

I see here something that I've never seen before.

I've been inside hundreds of AC658 Twins, and Fender's use of "mixed bias" {i dig that term, Tim!} is nothing new to me.

But this is the only time...ONLY time...i've ever seen the 150 ohm resistors equipped with bypass caps! That term strip soldered to the side of the chassis, over by the PT....that's new to me! I've never seen that before!

Have you guys seen that before?

mn

[Tim Swartz]

They all have the non-polarized 5uF cap between the cathodes... These are just mounted to the chassis. I've seen them like this on a few occasions. They are not really "cathode resistors" as they are not grounded. I'm not certain of their function, unless it is to provide a more consistent current flow???